The subject matter disclosed herein relates generally to an electrical insulation configuration for an electrical device, and, more particularly, to an electrical insulation configuration to provide electrical insulation between a power circuit and a user.
The use of ac-line-powered electrical devices, such as commercial devices, medical devices, and industrial equipment potentially exposes users of these devices to the possibility of electrical contact, burns, etc. due to the possibility of a breakdown in electrical insulation. As a result, many regulations from agencies ranging from the US FDA (Food and Drug Administration), European Commission, UL (Underwriters Laboratories Inc.), CSA (Canadian Standards Association) and other safety and regulatory bodies ensure that these devices comply with appropriate safety standards. For example, IEC (International Electrotechnical Commission) 60601-1 defines medical-equipment electrical-safety conditions necessary to protect patients, operators, and the surroundings, and IEC 61800-5-1 establishes electrical safety requirements for variable speed drives.
In electrical equipment, designers can isolate touch-safe low voltage circuitry from high-voltage circuitry using devices which provide electrical isolation such as optocouplers or transformers. The devices used to provide isolation between low voltage and high voltage circuits typically employ an insulation system based upon air spacings and solid insulation. When air is used as insulation between conductors such as component leads, the electrical safety standards require a minimum distance through air (clearance) and over an insulating surface (creepage). Likewise, when solid insulation such as plastic or ceramic is used between conductors, the standards require the insulating material to satisfy performance requirements determined by tests such as an AC voltage test, impulse test, or partial discharge test.
For example, for a 690V inverter, IEC 61800-5-1 requires 8 mm clearance between power circuits and the grounded chassis of the inverter; this is called “basic” insulation. However, devices such as optocouplers, when used to provide isolation between power circuits and low voltage circuits which are accessible to the user, must provide 14 mm clearance between the emitter and detector leads of the device. This increased insulation requirement is called “reinforced” or “double” insulation. A variety of ratings exist for optocouplers. For example, 8 mm, 14 mm, and larger optocouplers exist, but getting into the 14 mm size causes the optocouplers to be bulky and expensive.
It would, therefore, be desirable to provide a reinforced or double level of insulation using smaller, less expensive insulation devices, yet achieve a higher insulation rating.